Image forming apparatuses and sheet post-processing apparatuses include various trays, such as a processing tray for placing a sheet after image formation to bring the sheet to post-processing, and a receiving tray for receiving a discharged sheet after image formation or after post-processing. With a size reduction of the apparatuses, the processing tray and the receiving tray tend to have a shorter length in the sheet conveying direction. In other words, a sheet conveyed to a tray more easily slips on the tray because the tray only has a small area for supporting the conveyed sheet. Therefore, there arises a problem that the sheet conveyed to the tray goes out of alignment.
In particular, the processing tray included in a sheet post-processing apparatus is required to stack sheets in an accurately aligned manner in order to bind the sheets together after the sheets are discharged on the tray. JP-A 2007-137668 discloses a device which serves as means for maintaining alignment of the sheets on the processing tray, and which performs longitudinal alignment and crosswise alignment of the sheets.
The frictional force between a tray and a sheet is smaller than the frictional force between two sheets. Accordingly, a first sheet conveyed to the tray tends to slip on the tray more easily than a second or subsequent sheet conveyed to the tray. The first conveyed sheet is sometimes discharged obliquely on the tray, and is displaced. In addition, the first conveyed sheet is sometimes dragged to be displaced by the movement of the second or subsequent sheet.
As described above, the first sheet on the tray tends to be largely moved away from the intended position. There is a risk that the fist sheet may drop off from the receiving tray of an image forming apparatus or of a sheet post-processing apparatus. In addition, the processing tray of a sheet post-processing apparatus cannot completely eliminate misalignment of the sheets only by performing sheet alignment disclosed in JP-A 2007-137668, and problematic post-processing operations such as stapling are performed on the sheets still misaligned.